The present invention relates to a device for monitoring the depletion of bulk paper rolls, in particular as applicable to wrapping machines. Conventionally, the wrapping papers and/or sheer thermoplastic film materials utilized for packaging commodities by machine, and in particular by the machines which wrap cigarretts, are supplied wound in rolls or onto spools often of considerable dimensions.
In order to avoid stoppages occasioned by the roll of film or paper running out, these machines are usually provided with a work station incorporating two parallel pivots, each coaxially and rotatably supporting a relative roll; while one of the rolls is uncoiled and the continuous strip material fed to a cutting device for separation into discrete lengths as required by the wrapper assemblies of the machine, the other provides a reserve supply, ready to be connected up to the wrapping line the moment that the current roll is exhausted.
The depletion of the strip is monitored, and the empty roll ultimately replaced, by a conventional device including an arm positioned above the roll itself and pivoted at one end to a release mechanism. The remaining end of the arm carries an idle wheel or roller riding in contact with the outer circumference of the roll, generally under its own weight.
The device thus remains continuously in contact with the rolled strip material, and the gradual reduction in bulk of the roll causes the arm to drop until reaching a given angle in relation to its starting position, which corresponds to a given minimum radius from the center of rotation of the roll (calculated and set by the operator according to the size of bulk roll utilized). At this point, the arm activates the release mechanism, e.g. by means of a switch associated with its pivoted end, and sets the changeover operation in motion.
A device featuring improvements on the above described basic design is disclosed in EP 155 020. As disclosed in that document, the roll is fitted at center with an element generating a magnetic field, and use is made of an arm similar to that aforementioned but carrying a magnetic sensor at the end offered to the strip. The sensor is connected directly to the release mechanism, which must intervene to prevent further rotation of the empty roll, and set to respond when exposed to a predetermined magnetic field value substantially reflecting the approach of the middle of the roll, by relaying a signal to the mechanism. Both the devices above mentioned have drawbacks, from engineering and economic standpoints alike.
In particular, the first device (basically a mechanical system) is notably imprecise in sensing the end of the strip uncoiling from the roll. More exactly, the sensing operation can be rendered false both by possible imperfections in the shape of the roll (which may ovalize either as a result of being dropped, or more especially, as a result of being wound onto a core or center less than perfectly cylindrical in shape), and by the fact that monitoring is indirect and effected through intermediate mechanical media.
Under these conditions, the prescribed release and changeover angle may be reached accidentally early by the arm, resulting in a waste of strip material that could in fact have been utilized.
The second device partly overcomes the drawbacks just mentioned, but is conditioned economically by the fact that each roll has to be fitted with a source from which to generate the magnetic field. What is more, the value of the magnetic field itself can be influenced by external factors; in the event of the field being enlarged, the reading returned by the sensor will be false, and the roll once again taken out of operation with an excessive amount of strip jettisoned as waste instead of being used.